1. Field of the Invention
The present invention relates to an image display device, and more specifically, to a technology that is effective when display data is transmitted/received through a wireless transmission path.
2. Description of the Related Art
In an active matrix display device which is typified by the liquid crystal display device, a thin film transistor (hereinafter referred to as a TFT) composed of an active element is formed for each pixel, and a video is displayed by making each pixel storing display information.
On the other hand, a TFT formed using a poly-silicon film that is polycrystallized by processing laser anneal on an amorphous silicon film so that the mobility is raised to about 100 cm2/V·s as a semiconductor layer is called a poly-silicon TFT.
Since a circuit constructed with this poly-silicon TFT operates with a signal of a few MHz to a few tens of MHz at maximum, a data driver circuit for generating a video signal and a scanning circuit for performing scan as well as the pixels can be formed on a board of a liquid crystal display device etc. by the same process as that of the TFT constituting the pixel.
Further, as a communication system of transmitting/receiving a signal (e.g., display data, a display control signal, etc.) through the wireless transmission path, there can be considered a systems that use electrostatic induction (capacitive coupling), electromagnetic induction, and an electromagnetic wave. Incidentally, the electrostatic induction (the capacitive coupling) is disclosed by, for example, the following JP-A-2005-301219.
Generally the liquid crystal display panel consists of plural pixels arranged in a matrix (also called sub-pixels) and a drive circuit for inputting a drive voltage into the pixels. Here, an area where the plural pixels are arranged in a matrix is called a display area, and an area other than it where the drive circuit, etc. are arranged is generally called a frame part.
FIG. 19 shows a configuration of the pixels of an active matrix liquid crystal display panel. Each of the pixels arranged in a matrix consists of a thin film transistor (TFT) constituting an active element, a pixel electrode (PX), a counter electrode (CT), and a liquid crystal sandwiched between the pixel electrode (PX) and the counter electrode (CT).
By supplying a selection scanning voltage to a gate line (GL), turning on the thin film transistor (TFT) one display line by one display line, and supplying an image voltage sequentially to a driver line (DL) in accordance with it, it is possible to send an arbitrary image voltage to the pixel electrode (PX) of the each pixel. The counter electrode (CT) is one piece of a tabular electrode opposed commonly to all the pixels, and is connected to a common voltage (Vcom). The drive circuit has an interface function with external devices, a function of outputting the display data to the driver line (DL), and a function of driving the gate line (GL) in the liquid crystal display panel.
In the design of the liquid crystal display panel, importance is attached to reducing the frame part that does not participate in display.
On the other hand, although a method using an electromagnetic wave or electromagnetic induction as a communication system of transmitting/receiving the signal through the wireless transmission path is suited to longer-distance communications, the display signal must be modulated and demodulated using a carrier wave of a higher frequency; therefore, the signal needs to be modulated at a far higher frequency than a transmission rate of the signal. This constraint incurs electric power increase by high-speed circuit driving and increase of the area by formation of circuit elements.
On the other hand, although the capacitive coupling as shown in FIG. 5 of the above-mentioned JP-A-2005-301219 is limited for shorter distance transmission, it can be constructed only with electrodes for transmission because it does not need modulation and demodulation. Since circuits of modulation and demodulation are unnecessary, it has an advantage that the electrodes can be constructed with only a small occupied area.
However, in order to construct the coupling electrostatic capacitance for transmitting a data signal and an electric power signal, transmitting and receiving electrodes having a certain degree of area is needed. As one example of the area, it needs to be about 2 mm×2 mm for the data signal and about 2 mm×50 mm for the electric power signal.
With the conventional technology, these receiving electrodes must be arranged in the frame part outside the display area in a receiving-side liquid crystal display panel.
The reason is, firstly, because it is difficult to newly construct the receiving electrode in the each pixel. This is because the each pixel has already the pixel electrode (PX) in it, and a manufacture process of the display panel become complicated in order to dispose the receiving electrode that overlaps it.
Secondly, even when the receiving electrode is successfully arranged, since the counter electrodes (CT's) exist, parasitic capacitance is generated between itself and the receiving electrode. Originally, the thickness of the liquid crystal is several microns and is very much thin as compared with 0.5- to 1-mm gap that is assumed for a distance between the transmitting and receiving electrodes. Therefore, there is a problem that parasitic capacitance far larger than the electrostatic capacitance which can be constructed between the transmitting and receiving electrodes arises and it is impossible to transmit the data signal and the electric power signal efficiently.
Due to this fact, with the conventional technology, the receiving electrode must be arranged in an outer peripheral part of the display area, which causes a problem that a frame area increases further by the amount of the electrodes in the display panel.